1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
In some instances, it may be desirable to copy data from one storage device to another. For example, if a host writes data to a first storage device, it may be desirable to copy that data to a second storage device provided in a different location so that if a disaster occurs that renders the first storage device inoperable, the host (or another host) may resume operation using the data of the second storage device. Such a capability is provided, for example, by a Remote Data Facility (RDF) product provided by EMC Corporation of Hopkinton, Mass., e.g., Symmetrix Remote Data Facility (SRDF). With RDF, a first storage device, denoted the “primary storage device” (or “R1”) is coupled to the host. One or more other storage devices, called “secondary storage devices” (or “R2”) receive copies of the data that is written to the primary storage device by the host. The host interacts directly with the primary storage device, but any data changes made to the primary storage device are automatically provided to the one or more secondary storage devices using RDF. The primary and secondary storage devices may be connected by a data link, such as an ESCON link, a Fibre Channel link, and/or a Gigabit Ethernet link. The RDF functionality may be facilitated with an RDF adapter (RA) provided at each of the storage devices.
RDF may be used to provide backup systems for disaster recovery where one or more backup sites are maintained as mirrors of a primary site using RDF. When the primary site fails, work may be resumed at a backup site. Note, however, that different types of RDF transfers may be used with different tradeoffs for each. Synchronous RDF (SRDF/S) provides the most current version of the data, but often requires close proximity of the sites since data written to a primary site is not acknowledged until the data is written to the backup site. Close proximity (e.g., within same geographic area) may be undesirable for a disaster recovery system since there is a higher probability that a single disaster can cause both sites to fail. On the other hand, asynchronous RDF (SRDF/A) does not require close proximity of the sites, but the copy of the data at the backup site is usually delayed by a significant amount of time (e.g., five minutes), which may be unacceptable or undesirable in some instances.
The deficiencies of both SRDF/S and SRDF/A are addressed by a system that uses both, which is described in U.S. Patent Publication 2006/0069887 to LeCrone, et al. titled “TRIANGULAR ASYNCHRONOUS REPLICATION”, which is incorporated by reference herein. A drawback of triangular asynchronous replication is that it requires the synchronous R2 device to be passive so that no writes to the R2 device are allowed except in connection with maintaining the synchronous mirror. If it is desirable to use systems that provide mirroring and allow host writes to both devices (called “active/active systems”), then triangular asynchronous replication cannot be used.
Accordingly, it is desirable to provide a system that allows for providing an asynchronous backup site of active/active synchronous sites. Such a system provides the synchronous and asynchronous backup sites that are useful in a triangular replication system while still allowing modifications to both of the synchronous sites.